1. Field of the Invention
The present invention relates to a detection device in which electrodes are connected to each other through a bump, a photodiode array, and a method for manufacturing the detection device.
2. Description of the Related Art
In a detection device including a photodiode array (PDA) formed in a compound semiconductor and a read out IC (ROIC), a read out electrode of the read out IC and an electrode of the photodiode array face each other and are electrically connected to each other through a bump that lies therebetween. In a near infrared region or an infrared region that covers a wavelength longer than that of a visible region, a photodiode array is formed of a compound semiconductor and thus such a detection device is sometimes called a detection device having a hybrid configuration of a compound semiconductor and silicon (IC). Since an epitaxial layered body of the compound semiconductor is vulnerable to mechanical strength, the bump is composed of indium (In), which is soft and has a low melting point, in most cases.
When an electrode of a photodiode array is connected to a read out electrode of ROIC using a indium bump, heating to a high-temperature range is not preferable and thus thermal compression bonding that can be performed under low-temperature heating is employed. Herein, an oxide film is easily formed on the surface of indium, which causes poor electrical conduction, the generation of noise due to an increase in contact resistance, and the detachment between the photodiode array and the ROIC.
To solve the above-described problems, many proposals have been made. For example, to prevent poor contact caused by an oxide film of an In bump in the above-described hybrid configuration, there has been proposed a bump having a layered structure of (indium/gallium/indium) (Patent Document 1: Japanese Unexamined Patent Application Publication No. 3-276750).
To prevent detachment at a connecting point when a hybrid-type infrared image pickup device using an indium bump is manufactured, there has been proposed a method in which metal plating is performed on an electrode of a photodiode array (Patent Document 2: Japanese Unexamined Patent Application Publication No. 5-335375).
Furthermore, it has been proposed that, in the case where an indium bump is formed by vapor deposition, indium is caused to be easily melted by adding a substance that decreases a melting point to indium (Patent Document 3: Japanese Unexamined Patent Application Publication No. 5-136147).
In this method, when a resist pattern is lifted off, an indium alloy on the resist film can be easily separated from an indium alloy on an electrode, whereby an indium bump having a desired shape can be formed on the electrode.
There has been also proposed a method for forming an In bump on a ground metal layer by performing electrolytic plating on an InP wafer using a resist pattern having an opening formed in a portion corresponding to an electrode (Non-Patent Document 1: J. Jiang, S. Tsao, T. O'Sullivan, M. Razeghi, G. J. Brown, “Fabrication of indium bumps for hybrid infrared focal plane array applications”, Infrared Physics & Technology 45 (2004) 143-151).
The electrode surfaces of the photodiode array and the ROIC are each constituted by a single flat surface or several flat surfaces. The surface that contacts the indium bump is a flat surface having the maximum area. The electrode surface constituted by the flat surfaces is easily influenced by an oxide film or an impurity such as an organic matter contamination that inhibits the connection. Thus, the contact resistance is easily increased, which causes a variation in the performance between elements and between pixels.
In the method for forming an In bump by electrolytic plating proposed in Non-Patent Document 1, the outer layer of the InP wafer is damaged when the ground metal layer is removed, and charged particles of Ar or the like used in dry etching are left. When the method for forming an In bump is applied to the side of a read out circuit, a circuit element is degraded or becomes abnormal and thus a malfunction is caused on the detection device.